This invention concerns ion chamber type X-ray detector arrays. More specifically, this invention relates to structures for increasing the probability of detection of X-ray photons which interact with a detector gas in a region between ion chamber electrodes and a grounded input window.
A copending patent application by Nathan R. Whetten and John M. Houston, Ser. No. 616,930, filed Sept. 26, 1975, now U.S. Pat. No. 4,031,396 and assigned to the assignee of this invention describes an array of ionization chamber type X-ray detectors for use in determining the spatial distribution of X-ray photons in computerized tomography systems. The array comprises a plurality of substantially parallel, planar anodes separated by parallel, planar cathodes and enclosed in a gas of high atomic weight at a pressure in the range from approximately 10 atmospheres to approximately 50 atmospheres. X-ray photons interact with the gas to produce photoelectron-ion pairs. In the presence of an electric field, the electrons thus produced are collected on the anodes and the ions on the cathodes to produce electric currents in proportion to the X-ray intensity in the vicinity of those electrodes.
The anodes, cathodes and detector gas of such ion chamber arrays are typically enclosed in a metal pressure vessel having at least one thinned wall or window which is relatively transparent to X-ray photons. In a preferred embodiment of that ion chamber array, the window comprises a thin aluminum sheet disposed perpendicular to and spaced from approximately 0.5 millimeter to approximately 1.5 millimeters away from the collector electrodes (i.e., cathode and anode).
The electric fields in those, prior art ion chamber arrays are such that the electron-ion pairs produced in the space between the collector electrodes and the window fail to reach the detector plates and thus, do not contribute to the electrical output signal. In an array filled with xenon gas at a pressure in the range from approximately 20 atmospheres to approximately 30 atmospheres as much as five or ten percent of 60 kev X-rays passing through the window are absorbed in this "dead space"; a factor which significantly lowers the quantum detection efficiency and noise equivalent absorption of the array. The X-ray dose which must be administered to a patient undergoing examination in a system incorporating this detector is, therefore, substantially increased by the presence of the "dead space".